1. Field of the Invention
The present invention relates to an ultrashort pulse laser apparatus using an optical fiber, and to a terahertz measuring apparatus using, as an excitation light source, a laser beam emitted from the ultrashort pulse laser apparatus.
2. Description of the Related Art
Recently, a nondestructive sensing technique has been developed which employs electromagnetic waves (30 GHz to 30 THz; hereinafter referred to simply as “THz” waves) ranging from a millimeter wave to a terahertz (THz) wave. An imaging technique to realize a safe seeing-through apparatus instead of an X-ray fluoroscope has been developed as one application field of the electromagnetic waves in such a frequency band. As other application fields, there have also been developed a spectroscopic technique of measuring an absorption spectrum and a complex permittivity inside a substance to examine the coupling state, etc., a technique of analyzing biological molecules, and a technique of evaluating a carrier density and mobility.
In view of the importance in nondestructively performing a quality check in a production line of a factory, for example, applications to inspection of defects and/or foreign substances in moldings, and inspection of components, foreign substances and/or defects in chemical substances, etc. have also been studied. In those applications, a tomographic image can be taken by utilizing transmissivity of the THz waves. The chemical substances include, for example, inks such as pigments and dyes, toner, medicines, cosmetics, and paints. As an example of the inspection apparatus, Japanese Patent Laid-Open (Translation of PCT Application) No. 2006-526774 discloses an apparatus that is applied to the inspection of medicines by using THz-TDS (Time Domain Spectroscopy). With the disclosed apparatus, the kinds of materials inside medicines, for example, can be analyzed on the basis of spectrum information in the THz-wave region.
In the THz-TDS, as disclosed in Japanese Patent Laid-Open (Translation of PCT Application) No. 2006-526774, an excitation light source for generating and detecting THz waves is required which has a pulse width of about 100 femtosecond (fs) or less. A pulse laser using a titanium-sapphire crystal, for example, is preferably used as the excitation light source.
Such a laser using a solid crystal is advantageous in increasing an output, but it is not satisfactory in output stability and productivity. Further, that laser is very expensive. As a practical light source, therefore, the use of a fiber laser is studied.
The fiber laser is advantageous in that a very stable fiber amplifier can be used as a gain medium and a size can be reduced because of no necessity of constructing a spatial optical system. In addition, stability can be increased because the number of parts requiring adjustment of an optical axis is greatly reduced, and the cost can be reduced because productivity is increased.
The excitation light source used for the THz-TDS is required to have an average optical output of not smaller than several tens mW, desirably not smaller than 100 mW. It is difficult to constitute the fiber laser having such a high output and ultrashort pulse by using only a fiber oscillator. Therefore, that type of fiber laser is usually realized by connecting an output of an oscillator, which emits a seed light, to an external fiber amplifier and an external fiber compressor. As an example of the related art, Japanese Patent No. 2711778 describes an apparatus in which, after amplifying a seed light by a rare-earth doped fiber amplifier with normal dispersion, a pulse is shortened with dispersion compensation by using an anomalous dispersion fiber. Further, Japanese Patent No. 3811564 describes an apparatus in which a seed light is amplified by a rare-earth doped fiber amplifier with anomalous dispersion, while a pulse is shortened on the basis of a nonlinear effect with the Raman solution compression.
It cannot be said, however, that the fiber laser used at present has an output sufficient to increase an output of the THz waves and to widen a Fourier frequency band for the purpose of enhancing an analysis ability of a THz-TDS measuring apparatus. In other words, when the band of a THz spectrum is to be expanded to 10 THz or above, an ultrashort pulse of approximately 10 fs is required as the width of a laser pulse in an optical domain. Up to now, however, it has been difficult to realize a fiber laser having an output of not smaller than 100 mW and outputting such an ultrashort pulse.
With the apparatus described in the above-cited Japanese Patent No. 2711778, when the amplified pulse has a large peak value, the anomalous dispersion fiber used as a dispersion compensation fiber may distort a pulse wavelength due to the nonlinear effect, thus causing phase noise and Raman scattering. In other words, there has been a room for a further improvement in the practical excitation light source for the THz-TDS. For such an improvement, as described in the above-cited Japanese Patent No. 2711778, a peak value of the pulse is reduced by suppressing an optical output with an attenuator, or by increasing a chirp amount by using a fluorinated-Er doped fiber to provide larger normal dispersion. This is because an ordinary quartz fiber, which is not fluorinated, has a limitation in an amount of dispersion shift. However, when the fluorinated fiber is used, a difficulty arises in stably fusing those fibers with each other for coupling between them. Such a difficulty cancels the advantages of reducing spatial joints, cutting the cost, and increasing stability, which are specific to the fiber laser.
On the other hand, the apparatus described in the above-cited Japanese Patent No. 3811564 employs, as a quartz fiber, an erbium-doped fiber amplifier in an anomalous dispersion region that is relatively easily realized, and utilizes the Raman soliton nonlinear compression. According to such an arrangement, however, as an output increases, a wavelength shift based on a Raman shift is increased and it becomes more difficult to suppress a side lobe, i.e., a pedestal, in a time-dependent waveform. When the THz waves are generated by using a photoconductive device or a nonlinear crystal, the occurrence of a shift of the central wavelength from the design value raises a problem of reducing conversion efficiency and causes noise, which is not desired for the measurement using the TDS, due to THz waves generated by the presence of a pedestal.
One conceivable solution to those problems is to provide small normal dispersion by using an ordinary quartz fiber and to perform compression in a downstream stage. However, when the chirp amount is increased, namely when the fiber length is increased, to overcome the problems with the apparatus described in the above-cited Japanese Patent No. 2711778, energy of the chirped light extends up to a zero-dispersion wavelength region, thus causing an undesired nonlinear effect, e.g., a four-wave mixing. Further, when the optical output increases, induced Raman scattering occurs in the longer wavelength side. As a result, a pedestal is generated in a time-dependent waveform after the compression.